JPS62509B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for driving a thin film EL element, and more particularly to a method for modulating brightness for displaying halftones. Further, the present invention provides a method that can be driven with a small number of circuit elements, and furthermore, the withstand voltage required of the circuit elements can be reduced. Although the structure and characteristics of thin film EL matrix elements have been explained in many patent applications filed by the applicant,
Let me explain briefly again. Thin film EL display devices are made of In ₂ O ₃ or In 2 O 3 on a glass substrate.
A transparent electrode of SnO ₂ is arranged in a striped pattern, and a dielectric material such as Y ₂ O ₃ , Si ₃ N ₄ , TiO ₂ , Al ₂ O ₃ is placed on top of the transparent electrode.
Furthermore, on top of this, for example, a fluorescent layer such as Mn-doped ZnS (yellow-orange luminescent), and on top of that, Y ₂ O ₃ ,
A dielectric material such as Si ₃ N ₄ , TiO ₂ , Al ₂ O ₃ is deposited to a thickness of 500 to 10,000 Å using thin film techniques such as vapor deposition or sputtering to form a double-insulated three-layer structure, and then A striped electrode made of Al is arranged in a direction perpendicular to the transparent electrode to form a matrix electrode. In a three-layer thin film EL display device having such a structure,
When one of the first electrode group 2 and one of the second electrode group are selected and an appropriate alternating current voltage is applied, a minute area portion sandwiched between the two electrodes intersects emits light. This corresponds to one picture element on the screen. By combining these, characters, symbol patterns, etc. are displayed. ELs with such a structure have superior characteristics in terms of brightness, lifespan, and stability compared to conventional distributed EL elements. The present invention relates to a method for driving the above three-layer thin film EL matrix element, and the present invention uses a constant current circuit and controls the operating time of the constant current circuit to control the voltage applied to the thin film EL element. It controls and modulates the brightness. A circuit according to an embodiment of the present invention is shown in FIG. 1, and will be described below. Reference numeral 10 denotes the thin film EL element, in which the data side (X) electrodes X ₁ to Xm are made of transparent electrodes 11;
, and only progressive scanning (Y) electrodes Y ₁ to Yn made of aluminum electrodes 12 are shown. A power supply circuit 20 supplies a write voltage Vs and a malfunction prevention voltage (Vs-Vp) to the thin film EL element 10 from the scanning electrode side. A switch _S1 is inserted between each of the above voltages and line A, which is turned on during a write operation. Also, the malfunction prevention voltage and line A are diodes.
Connected via _D1 . 30 is a scanning side selection switch circuit, which is connected to each scanning electrode.
Switches Sy ₁ -Syn are connected between Y ₁ -Yn and line A to sequentially select scanning electrodes. Reference numeral 40 indicates a diode circuit connected between the entire scanning electrode and the ground. A switch _S2 is inserted between the common line B of one of the diodes and the ground to constitute a write voltage discharge circuit and a refresh circuit. The common line C of the other diode is grounded via diode D ₂ to form a refresh voltage discharge circuit, and the switch S ₃
It is connected to the clamp voltage Vp through the voltage Vp to apply a voltage to prevent malfunction of half-selected picture elements. 50 is a data side selection switch circuit, and a constant current switch circuit is connected between each data electrode _X1 to Xm and ground.
Sx ₁ to Sxm are connected and selectively turned on as appropriate according to the displayed information. Reference numeral 60 indicates a diode circuit connected between all data electrodes and the sustain voltage Vs. A diode _D3 is connected between the diode common line D and the ground to constitute a write voltage discharge circuit, and is also connected to the refresh voltage Vr via a switch _S4 . Next, the operation of the circuit of the above embodiment of the present invention will be explained. FIG. 2 shows a time chart of the circuit of the present invention. First, all data switches are turned on at the first timing _t1 .
Sx ₁ ~ Sxm and selected scan electrode for writing
Turn on Yj switch Syj. That is, in this timing period, all picture elements on the selected scan electrode are charged with the malfunction prevention voltage (Vs-Vp). During charging, since the data switch is composed of a constant current element, the picture element is charged to a voltage (Vs-Vp) at a certain constant gradient according to the limitations of the constant current element. Since the thin film EL element is capacitive, this voltage is maintained even after the above switches are turned off. At the next timing _t2 , the writing picture element M(i,
Only the switch Sxi of the data electrode Xi and the switch Syj of the scan electrode Yj including data electrode Yj are turned on, and the other data switches Sxk and scan switch Syl are turned off. Then turn on switches S ₁ and S ₃ . Here, the data switch Sxi is turned on for a time corresponding to the degree of modulation, and a voltage proportional to this on time is applied to the write picture element M(i,j).
In other words, since the data switch is a constant current element,
When this current is I and the on-time is ti, the voltage Vij applied to the write picture element M(i, j) is expressed by the following equation. Vij = (Vs - Vp) + I・ti/n・C C: Capacitance value per picture element n: Number of scanning electrodes Therefore, by changing the on time ti, the voltage applied to the writing picture element can be changed. can do. Since the luminance of light emitted by a thin film EL element is proportional to the applied voltage, by controlling the voltage as described above, light is emitted with controlled luminance. It is desirable that the on-time of the data switch is controlled so that the end of the on-time coincides with the end of the second timing. This is so that the change in on time gives a change in brightness as pulse width modulation. In the case of this example, the on time of the data switch is short, so it is not much of a problem, but since the thin film EL element is a capacitive element,
Even after the data switch is turned off, the writing picture element maintains the writing voltage and cannot give brightness modulation.
If the end of the on-time coincides with the end of the second timing, the periods during which the voltage is held will all be the same, and pulse width modulation can be applied at the same time. Therefore, although Figure 2 shows the case where one data switch is selected, if there are many data switches to be selected, the on time of the data switch will change depending on the luminance, and the off time will remain the same. That is why. During this write drive, switch _S2 is turned on and clamp voltage Vp is applied to half-selected picture elements on the data electrode to prevent malfunction. At the third timing _t3 , the above switches _S1 , _S3 ,
Sxi and Syj are turned off and switch _S2 is turned on.
Then, diode D ₃ → diode circuit 60 →
All data electrodes → all scanning electrodes → diode circuit 40
→Switch S ₂ →A grounded circuit discharges the charge stored in the writing picture element at the second timing. The above operation is performed for the first scanning electrode Y1 to the nth scanning electrode _Y1 to
This is performed for the th scanning electrode Yn, and after one field of scanning is completed, refresh driving is performed. For refresh drive, switches S ₂ and S ₄ are turned on to apply a refresh voltage to all picture elements. Since the refresh voltage is equal to the write voltage Vs and is applied from the data electrode, it will be applied to the picture element with the opposite polarity, and the picture element to which writing has been performed will have the same brightness as the brightness at the time of writing. Emits light. Thereafter, all data switches Sx ₁ to Sxm are turned on, and discharge is performed in the circuit of diode D ₂ → diode circuit 40 → all scan electrodes, all data electrodes, all data switches → ground. The above-described write drive and refresh drive are performed for the number of frames per second. In addition, in FIG. 2, Yj is the voltage applied to the electrode Yj, Yl is the voltage applied to the electrode Yl (l≠j),
Xi represents the voltage applied to the electrode Xi, Xk represents the voltage applied to the electrode Xk, and M(i, j) represents the voltage applied to the picture element M(i, j). As described above, the present invention uses a constant current element to control the on-time of a data switch selected according to the luminance modulation degree, and to apply (Vs-Vp)+I・ti/
A voltage of nC is applied, and a voltage of Vr is applied during refreshing to apply a bipolar pulse. Therefore, although pulses with different amplitudes are applied to the thin film EL element, the thin film EL element emits light with a brightness corresponding to the average voltage of the applied amplitudes. Therefore

The on-time ti can be determined so that [Formula] is the required brightness. Here, during the above write drive, the first timing
The reason for applying the voltage (Vs - Vp) at t ₁ will be explained. Now, in a matrix panel with n scan electrodes and m data electrodes, there is no first timing of the present invention, and when scan electrode Yj is scanned, k data electrodes are selected and displayed with the same modulation factor. think of. That is, in FIG. 1, the switches S ₁ ,
S ₃ , Syj is turned on, switches Sx ₁ , Sx ₂ , ......
..., Sxk is turned on at the same time. The equivalent circuit at this time is as shown in FIG. 3 when each picture element is represented as a capacitor. In FIG. 3, if the capacitance per picture element is represented by C, the electrode resistance is 0, and the constant current value per data switch is I(A), each capacitor has the following meaning. C ₁ = kC C ₂ = (m-k) C C ₃ = (m-k) (m-1) C C ₄ = k(n-1) C where m: number of data electrodes n: number of scanning electrodes k: Number of selected picture elements when driving one scanning electrode Under these conditions, the voltage V applied to the capacitor _C1 of the selected picture element t seconds after the data switch is turned on is as follows. (Vs-Vp) (m-k) < nkVp When Γt≦tp (tp is the time until the connection point p of capacitors _C3 and _C4 reaches voltage Vp) V=It(m+nk-k)/mnC Γts >t>tp (ts is the time when the potential at point g in Figure 3 becomes 0) (ts=C/I{Vs+(n-1)Vp}) V=(Vs-Vp)(n-1 )/n+It/nC When Γt≧ts V=Vs (Vs-Vp) (m-k)≧nkVp When Γt<ts (ts is the potential of point g in Figure 3 is 0)
(ts=VsmnC/I(m+nk-k)) V=It(m+nk-k)/mnC When Γt≧ts V=Vs The conditions as above (i.e., the number of scanning electrodes, The time at which V=Vs occurs varies depending on the number of data electrodes, the number of data, and changes in driving voltage. This is very inconvenient for a device that attempts to modulate the luminance of emitted light as in the present invention. In particular, the applicant filed a patent application on May 30, 1975.
When using the high-voltage MOS IC filed in No. 66200, ``High-voltage field-effect semiconductor device,'' as a data switch, it becomes a problem because the high-voltage MOS IC has a small constant current value. On the other hand, in the above embodiment of the present invention, all picture elements on the selected scanning electrode are charged with voltage (Vs - Vp) in advance at the first timing, so in the equivalent circuit of Fig. 3, the capacitor C ₁ , _C2 has a voltage (Vs−
Vp) is being charged. Therefore, the second
When the write voltage Vs is applied at the timing, no current flows through the capacitor _C2 due to the voltage Vs, so the voltage V applied to the capacitor _C1 t seconds after the data switch is turned on is as follows. Γt<ts (ts is the time when the potential at point g in Figure 3 becomes 0) (ts=VpnC/I) V=(Vs-Vp)+It/nC Γt≧ts V=Vs In this way, the selected picture element The time required for the voltage applied to _C1 to reach the write voltage Vs does not change depending on the number of data and remains constant. Therefore, the driving method of the above embodiment of the present invention is a suitable method when using a high voltage MOS IC. In addition, if all pixels on the selected scan electrode are not charged with the voltage (Vs - Vp) in advance, the half-selected pixel on the selected scan electrode at t≧ts (as shown in Figure 3).
The voltage applied to C ₂ , C ₄ ) is as follows. Now, considering the case of Vp = 0, C ₂ →k(n-1)/nk+m-kVs C ₄ →(m-k)/nk+m-kVs In this way, the voltage applied to the half-selected pixel is the matrix panel It will change depending on the number of electrodes and the amount of data (k). For example, when m=5n and k=m/2, a write voltage Vs is applied to the capacitor _C2 , causing the half-selected picture elements to emit light. However, if the voltage Vp is set to an appropriate value, approximately the voltage (Vs-Vp) is applied to the capacitor _C2 , and the voltage Vp is applied to the capacitor _C4 , so that the half-selected picture elements can be prevented from emitting light. However, when m=5n and k=1, a voltage of approximately 5/6Vs is applied to the capacitor _C4 , and the half-selected picture element emits light. In this case, point p in FIG. 3 has a potential higher than voltage Vp, which is the same as Vp=0. In order to prevent this half-selected picture element from emitting light, it is sufficient to configure a circuit that applies voltage Vp to all data electrodes via a diode separation circuit during write drive, but the number of circuit elements increases accordingly. On the other hand, in the above embodiment of the present invention, since the voltage (Vs - Vp) is charged in advance, the capacitors C ₂ and C ₄ of half-selected picture elements are The voltage applied to the capacitor C ₂ is (Vs - Vp), and the voltage applied to the capacitor C ₄ is Vp. By selecting the voltage Vs appropriately, the voltage applied to the capacitors C ₂ and C ₄ of the half-selected picture element is determined by the voltage applied to the light emission of the thin film EL element. If the voltage is set to be below the threshold voltage, half-selected picture elements will not emit light. Further, a circuit for preventing light emission of half-selected picture elements is not required. In the above embodiment, the voltage (Vs - Vp) is charged at the first timing, but if the on-current of the data switch can be made sufficiently large, the voltage to be charged in advance can be (Vs - Vp). It is not necessary. Furthermore, in the driving method of the present invention, when the switch S ₁ is on during writing, the switch S ₃ is always on, so the breakdown voltage of the scanning switches Sy ₁ to Syn and the diodes constituting the diode circuit 40 is ( Vs−Vp) is sufficient. Further, the diode constituting the diode circuit 60 may also have a withstand voltage of voltage Vs.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment for carrying out the driving method of the present invention, FIG. 2 is a time chart of the circuit of FIG. 1, and FIG. 3 is an equivalent circuit diagram explaining the operation of the present invention. 10: thin film EL matrix element, 20: power supply circuit, 30: scanning side selection switch circuit, 40: diode circuit, 50: data side selection switch circuit, 60: diode circuit.

Claims (1)

[Claims] 1. In a method for driving a thin film EL element in which a thin film dielectric layer sandwiching a thin film EL layer is interposed between mutually orthogonal matrix electrodes, a constant current is supplied to a writing picture element, and the supply time of the constant current is A method for driving a thin film EL element, characterized in that brightness is modulated by controlling the charging voltage of the writing picture element by controlling according to a video signal.